1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine for varying the valve timing of one of or both of an intake valve and an exhaust valve of the internal combustion engine, and to an internal combustion engine including the variable valve timing control apparatus. In particular, the present invention relates to an improvement in reducing or canceling shock that occurs when the valve timing varies.
2. Description of the Background Art
In recent years, among internal combustion engines (engines) incorporated into vehicles, an increasing number have employed variable valve timing apparatuses for varying the valve timing of intake valves and exhaust valves with the object of increasing output, reducing fuel consumption, and lowering exhaust emissions.
As disclosed in Japanese Patent Laying-Open No. 2004-245192, for example, most variable valve timing apparatuses in use at present drive a phase-varying mechanism with a hydraulic pressure from a hydraulic pump that actuates with rotary drive power from a crankshaft to vary the rotation phase of a camshaft with respect to a crankshaft, so as to vary the valve timing of intake valves and/or exhaust valves driven to open and close by the camshaft.
However, with such a hydraulically driven variable valve timing apparatus, in some cases, in cold conditions and on starting of the engine the hydraulic pressure is insufficient or the responsiveness of hydraulic control falls, and therefore there is a disadvantage that the accuracy of valve timing control is not fully achieved.
In this connection, as disclosed for example in Japanese Patent Laying-Open No. 6-213021, motor-driven variable valve timing apparatuses have been developed, in which a phase-varying mechanism is driven with drive power from a motor to vary the rotation phase of a camshaft with respect to a crankshaft and thereby vary a valve timing.
Although in the aforementioned motor-driven variable valve timing apparatus there is an advantage that the response speed is fast, this high response speed may possibly cause a sharp variation in the output torque of the engine when the valve timing is varied. Due to this variation in torque, sometimes shock (vibration) occurs with the vehicle body and let the passenger feel uncomfortable. For example, when advancing valve timing in accordance with an increase in engine load, the valve timing shifts to advance side greatly in a short period even in a situation where the driver gradually increases an accelerator opening degree. This instantaneous variation in the valve timing involves a variation in the combustion state in the combustion chamber, which in turn invites a sharp variation in torque. Thus, shock (vibration) occurs with the vehicle body.
As above, the motor-driven variable valve timing apparatus invites the trouble which has not been invited by the hydraulically driven variable valve timing apparatus that uses a hydraulic pump actuated by the rotary drive power of the crankshaft.
In order to solve the trouble, it may be possible to configure the variable valve timing apparatus by placing a certain upper limit on a valve timing varying rate so that the valve timing is always varied at a low rate with which shock does not occur with the vehicle body. For example, the maximum value of the rotation speed of the motor that is the drive source for varying the valve timing may be set low.
However, the valve timing then varies slowly even in a situation where the driver abruptly increases the accelerator opening degree, that is, where the driver requires quick acceleration. It takes a long time to attain the valve timing commensurate to the driver's requirement, and the engine response is not fully attained. Thus, full use of the true capability of the motor-driven variable valve timing apparatus is not made.
It should be noted that such a trouble is not limited to the motor-driven variable valve timing apparatus, and it may occur with any variable timing apparatus that is higher in response speed of the valve timing as compared to the aforementioned hydraulically driven variable valve timing apparatus (for example, the one having an electrohydraulic pump as the drive source).